Inorganic ceramic paper, its method of manufacturing and articles produced therefrom

ABSTRACT

An inorganic ceramic paper includes inorganic ceramic fibers and an inorganic binder system of fibers consisting essentially of silica. The binder system initially includes organic materials as well as the silica fibers, however, the organic materials are burned out of the ceramic paper. The silica fibers remain and flow so as to maintain the structure and integrity of the ceramic paper and hold the ceramic fibers together. Such a ceramic paper is useful in a variety of applications and is particularly useful as a filter in the inflator unit of an automotive airbag.

This application is a divisional of application Ser. No. 08/156,378,filed Nov. 22, 1993, now U.S. Pat. No. 5,419,975.

TECHNICAL FIELD

The present invention relates generally to ceramic paper and its methodof manufacture. More particularly, the invention relates to inorganicceramic papers for use in high temperature environments having highstrength and good flexibility, porosity, and rollability properties.Specifically, the invention relates to inorganic ceramic paper boundtogether by silica fibers and being free of all organic materials. Suchceramic paper is especially useful in the manufacture of air bagfilters.

BACKGROUND OF THE INVENTION

Ceramic paper generally refers to high temperature resistant, insulatinginorganic sheet material having a thickness of up to 1/4 of an inch andpredominantly comprising ceramic fibers. Such materials are referred toas "paper" because of their resemblance to wood pulp papers and becausethey can be rolled on conventional papermaking machines. Such paper isused for a wide variety of household and industrial applications.Recently, the automotive industry has become interested in ceramic paperfor use in the inflator filter units of automotive airbags.

In particular, the automotive airbag manufacturers are attempting tomake more efficient use of the propellant or generant, which is veryexpensive, by designing inflators which burn hotter. The hotter burninginflators result in optimizing reaction completion which, in turn,allows the airbag manufacturer to use less propellant or generant in theinflator, and thus, reduce cost.

However, such a very high output inflator is useless without acompatible ceramic paper filter material which is strong enough,flexible enough, and porous enough to withstand the high output blast towhich it is subjected. Furthermore, any such ceramic paper filtermaterial must be free of organics so that outgassing does not add to theinternal combustion pressure and temperature or to the toxicity of thegas.

More specifically, an inflator filter unit of ceramic paper desirablyshould have a minimum standard for tensile strength of at least about195 gm/cm (500 grams/inch), preferably 315 gm/cm (800 grams/inch) andminimum low pressure/high flow rate of about 1.84 cubic meters perminute (65 cubic feet per minute). Also, any such filter unit should beorganic-free (0% Loss On Ignition) and exhibit no dustiness.

Heretofore, ceramic papers relied principally on latex, polyvinylacetate and phenolic resin type binders for achieving required tensilestrength properties. However, as noted hereinabove, upon exposure tohigh temperatures, these organic binders would frequently burn out,causing outgassing which would result in a loss of tensile strength forthe paper and the release of combustion products into the atmosphere,and when used in airbag inflators, would increase the temperature andpressure to variable levels, a result which is not desired. Air baginflator filters today employ at least some organic binder material,typically, about three percent (3%). However, even this rather lowamount of organic binder may create variations in temperature andpressure.

In an effort to overcome this problem of outgassing, one alternativewhich has been tried was the production of an inorganic sheet withoutorganic binders. However, virtually all attempts to prepare inorganicceramic papers have so far proven unsuccessful in one way or another.Some attempts yielded very weak products, e.g., burn grade papers, whileothers resulted in formulating papers with limited flexibility androllability. For example, Barber, Jr. U.S. Pat. No. 5,053,107 provideshigh temperature resistant, insulating inorganic ceramic papers for usein high temperature environments which do not include any organicbinders. Instead, the ceramic paper produced in this patent is heldtogether by fiberglass fibers such as E glass, ranging in length from100 μm to 0.75 inches in place of the organic binders. In fact, thefiberglass fibers are actually melted to the ceramic fibers. Notably,however, these ceramic papers become brittle at temperatures greaterthan 1400° F. and are otherwise not easily rolled. Moreover, thisceramic paper is not nearly as flexible as is required for use ininflator units.

Thus, while substantial effort has been made to prepare organics-freepapers with high tensile strength, good flexibility, porosity and flowproperties, retaining these properties in the same inorganic ceramicpaper at commercially acceptable levels has not previously beenachieved.

Accordingly, a need exists for a strong, porous, flexible and rollableceramic paper which is free of all organic binders. Such ceramic paperwould be very beneficial to air bag manufacturers in the development ofan organics-free inflator filter unit.

SUMMARY OF INVENTION

It is therefore a primary object of the present invention to provide aninorganic ceramic paper which is free of organic materials at the timeof use.

It is another object of the present invention to provide an inorganicceramic paper, as above, which possesses the desired strength, porosity,flow, and flexibility characteristics for certain desired applications.

It is another object of the present invention to provide an inorganicceramic paper, as above, which conforms to the characteristics requiredfor use as an inflator filter unit in automotive airbags.

It is another object of the present invention to provide an inflatorfilter unit for use in airbags including an inorganic ceramic papermaterial.

It is a further object of the present invention to provide an inflatorfilter unit as above, which, because of its porosity, allows thegenerant or propellant to burn more efficiently and thus reduce the costof the unit.

One or more of the foregoing objects together with the advantagesthereof over the known art relating to ceramic papers, which shallbecome apparent from the specification which follows, are accomplishedby the invention as hereinafter described and claimed.

In general, the present invention provides an inorganic ceramic papercomprising inorganic ceramic fibers and an inorganic binder system offibers consisting essentially of silica, the ceramic paper being porousand free of all organic materials.

The present invention also provides inorganic ceramic paper including acomposition of inorganic ceramic fibers and an inorganic binder systemof fibers consisting essentially of silica produced by mixing theinorganic ceramic fibers, the silica fibers and a plurality of organicmaterials into a mixture or slurry; forming a ceramic paper mat from themixture or slurry; and burning out all organic material in the ceramicpaper mat, whereby the silica fibers hold the ceramic fibers together.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of an airbag module having an inflator filterunit according to the concept of the present invention.

PREFERRED EMBODIMENT FOR CARRYING OUT THE INVENTION

As noted hereinabove, the present invention is directed toward ceramicpapers which have high strength and good flexibility and which areorganics-free. Preferably, the inorganic ceramic papers disclosed hereinhave a tensile strength of at least about 500 grams per inch, preferablyabout 800 grams per inch, a flow rate (permeability) of at least 65cubic feet per minute, and 0 percent Loss On Ignition (LOI) exclusive ofwater vapor. Lower tensile strength can be tolerated if the paperpossesses good handleability, such as good elongation or elasticitywithout breakage. The permeability is also a feature of the porosity ofthe paper required for the applications set forth below. Specifically,flow rates of less than 65 cubit feet per minute are associated withnonporous or insufficiently porous paper, and therefore, will not workin the high-flow applications noted herein. Preferably, a flow rate ofat least 200 cubic feet per minute is desired.

The ceramic papers of the present invention have utility in a widevariety of applications and are especially useful as inflator filterunits in automotive airbags where high temperature resistance as well asstrength and flexibility are required. Additionally, the ceramic papersof the present invention appear to optimize filter efficiency andprovide little, if any, dustiness.

The inorganic ceramic papers of the present invention preferably have athickness ranging from about 0.0762 cm to 0.635 cm (or from about 0.03inches to 0.25 inches) and include inorganic ceramic fibers and aninorganic binder system of fibers consisting essentially of 100 percentsilica. However, the binder system is inorganic only as an end result ofthe process of producing the ceramic paper. That is, the ceramic paperis initially formed into a mat on-line with organic materials includedas binders. However, once the formation of the ceramic paper mat iscomplete, the ceramic paper is taken off-line and the organic materialsare burned out or annealed in ovens at extreme temperatures such thatall organic material is removed. Importantly, however, the ceramic paperdoes not break or fall apart. Instead, the paper is held together byinorganic fibers consisting essentially of 100 percent silica.Accordingly, as an end product, the ceramic paper is completelyorganics-free and includes as a binder, the fibers consistingessentially of 100 percent silica.

For the purposes of this invention, it should be understood that theterm "on-line" refers to the process steps which are employed on theproduction line for the general formation of the ceramic paper. Thesesteps may include mixing, weaving (if necessary), rolling and evenannealing, where necessary, usually at certain temperatures well below1000° F. These steps typically occur in the papermaking machine. Itshould also be understood that the term "off-line" refers to the processsteps which are not employed on the production line. Typically, thesesteps are performed after the ceramic paper has been produced, but notyet finished, and require the physical removal of the ceramic paper fromthe production line or papermaking machine. For example, in the presentinvention, the ceramic paper is initially formed from a composition ofinorganic fibers and a binder system which includes both inorganic andorganic materials. In order to provide the ceramic paper with a totallyinorganic binder system, the organic materials must be burned off. Thisis done by removing the ceramic paper from the production line andplacing the paper in an oven as detailed hereinbelow.

With respect to the initial production of the ceramic paper, inorganicceramic fibers are mixed with a plurality of different inorganic andorganic materials suitable for holding or binding the ceramic fiberstogether, so as to form a mixture or slurry. Any mixing means may beused, but preferably the fibrous components are mixed at about a 0.5%consistency or solids content (0.5 parts solids to 99.5 parts water)with Lightnin' Mixers in polypropylene stock chests. The slurry is thendiluted with water to enhance formation, and it is finally flocculatedwith binder and drainage retention aid chemicals. Then, the flocculatedmixture or slurry is place onto a papermaking machine to be formed intoa ceramic paper mat using no dandy rolls or presses. The mats or sheetsare formed by vacuum casting the slurry or mixture with conventionalpapermaking equipment and are typically dried in ovens generally heatedto 200° F. to 600° F. However, at this point the process of producingthe ceramic paper of the present invention is not completed. After themat is formed, it is taken off-line and placed in a high temperatureoven to burn out all of the organic materials in the mat. Desirably, theceramic paper is heated in the oven at a temperature of about 1000° F.to about 1500° F., preferably about 1300° F. to about 1500° F. for about2 to about 10 minutes, although the temperature and the time for heatingmay vary depending on the type of oven used.

Notably, the initial mixture components include inorganic ceramic fibersand a binder system having a plurality of different inorganic andorganic materials. More particularly, the composition may comprise aplurality of different grades of inorganic ceramic fibers. Desirably,about 10 to about 50 percent by weight bulk inorganic spun ceramicfibers, about 10 to about 80 percent by weight coarse-chopped inorganicspun ceramic fibers, and 0 to about 10 percent by weight blown ceramicfibers are used (percentages based on total weight of dried mixturesolids). Optimally, about 36 percent by weight bulk spun ceramic fibers,about 36 percent by weight coarse-chopped spun ceramic fibers, and 1percent by weight blown ceramic fibers are employed. Such ceramic fibersparticularly useful in making the ceramic papers of the presentinvention are commercially available from and marketed by TheCarborundum Company of Niagara Falls, N.Y., under the registeredtrademark Fibeffrax. These inorganic fibers are ceramic types producedfrom alumina and silica, such as vitreous aluminosilicate fibers.Alternative useful ceramic fibers include mineral wool fibers, zirconiafibers, zirconia tow, conventional silica fibers, alumina fibers andmixtures thereof. However, it should be understood that this list offibers is a representative sample only, and that other ceramic fibersmay also be suitable for the purposes and under the conditions detailedherein.

The binder system for the ceramic fiber mixture provides strength andflexibility to the composition and, most importantly, holds the ceramicpaper together. That is, the binder system incorporated therein helps tokeep the paper from cracking or falling apart during processing on thepapermaking machine, during burn out in the oven and thereafter, for thelife of the ceramic paper.

In order to make the ceramic paper crack resistant, a plurality oforganic binder materials may be provided initially as part of the bindersystem in the on-line formation of the ceramic paper. In the preferredembodiment, at least two organic latexes are employed. First, from about1 to about 5 percent by weight, and preferably about 2 percent by weightof an elastic type of latex material such as a modified acrylic with asmall amount of bonded nitrogen dispersed therein is provided whichpermits the ceramic paper to be processed on-line without cracking onthe papermaking machine. Suitable latex materials having thesecharacteristics are commercially available acrylic ester copolymers.

The second latex is a very stiff, flame retardant halogenated latexmaterial which is preferably added to the mixture in an amount rangingfrom about 2 to about 10 percent by weight, preferably about 4 percentby weight. This latex material is generally believed to control the rateat which the gas escapes from the ceramic fiber mat during the off-lineburn out stage in order to maintain the thickness and integrity of theceramic paper during this process. Suitable latex materials having thesecharacteristics are commercially available chlorinatedvinylidene/butadiene latexes.

However, in order for the ceramic paper to remain in one piece and notto fall apart after the burn out stage, an inorganic binder materialcapable of withstanding the extreme temperatures within the ovenutilized in the burn out stage must also be present. One such inorganicbinder material includes fibers consisting essentially of 100 percentsilica. These fibers have an average length ranging from about 1 toabout 3 inches and generally comprise from about 1 to about 50 percentby weight of the ceramic paper mixture solids. Importantly, as notedherein, the silica fibers are not burned out of the composition like theorganic materials. Instead, the silica fibers tend to flow in a mannerwhich allows them to hold the ceramic fibers together. Moreover, thesilica does not become brittle when dried, and therefore, does not cracklike other inorganic binder materials. It stays flexible even atannealing temperatures and is not as dusty as are many other types offibers. Specific representative examples of useful fibers consistingessentially of 100% silica are produced by BP Chemicals (Hitco) Inc. ofSanta Ana, Calif., under the trademark Refrasil®, as well as by SumitomoCorp. of Japan under the trade name Sandtex, a product commerciallyavailable through ordinary channels of commerce. Such silica fibers maybe produced from standard glass fibers such as E-glass, by treating theglass fibers in an acid bath such that every other material is leachedfrom the fibers except silica.

In addition, other ingredients commonly used in producing ceramic papersmay be included in the production thereof. Specifically, ingredientssuch as alum, a drainage retention aid polymer, and a dispersant may beincluded. It should be understood, however, that other commonly usedingredients may be employed without departing from the spirit of theinvention as claimed hereinbelow.

As commonly known, alum is used to precipitate the organic latexes ontothe surface of the fibers. In the presence of alum, the latex which isnegatively charged readily coats the fibers. Varying levels of alum mayalso be used to impart more rigidity or flexibility to the paper mats.Preferably, from about 0.5 to 2 percent by weight alum is used. Adrainage retention aid polymer, a positively charged polymer, then pullsthe negatively charged coated fibers together and allows any free waterto be removed. One such drainage retention aid polymer commerciallyavailable from Nalco Chemicals of Naperville, Ill., is Nalco 7530, anacrylamide modified cationic copolymer. Preferably, from about 0.1 to0.4 percent by weight drainage retention aid is employed. A dispersantmay also be used to prevent the ceramic fibers from breaking apart suchas by aiding mixing through the neutralization of static charges in thesystem. One such suitable dispersant is commercially availablepolyethylene oxide. Preferably, from about 0.05 to 0.2 percentdispersant is used.

In order to demonstrate the manufacture of the completely inorganicceramic paper, the following formulation was prepared and made into aceramic paper on a rotoformer papermaking machine.

                  TABLE 1                                                         ______________________________________                                        Ingredient             Weight Percent                                         ______________________________________                                        Coarse-Chopped Spun Ceramic Fiber.sup.a                                                              36                                                     Bulk Spun Ceramic Fiber.sup.b                                                                        36                                                     Blown Ceramic Fiber.sup.c                                                                            1                                                      100% Silica Fiber.sup.d                                                                              20                                                     Halogenated, Flame Retardant Latex.sup.e                                                             4                                                      Elastic Processing Latex.sup.f                                                                       2                                                      Alum @ 48.8 wt %       0.65                                                   Drainage Retention Aid Polymer.sup.g                                                                 0.25                                                   Dispersant.sup.h       0.1                                                    Total                  100                                                    ______________________________________                                         .sup.a 7010 CoarseChopped RCF available from The Carborundum Company,         Fibers Division, Niagara Falls, N.Y.                                          .sup.b 7010T Bulk RCF available from The Carborundum Company, Fibers          Division, Niagara Falls, N.Y.                                                 .sup.c 7000 AB RCF available from The Carborundum Company, Fibers             Division, Niagara Falls, N.Y.                                                 .sup.d Refrasil F100A200 silica fiber                                         .sup.e Chlorinated vinylidene/butadiene latex.                                .sup.f Acrylic ester copolymer latex.                                         .sup.g Acrylamide modified cationic copolymer.                                .sup.h Polyethylene oxide.                                               

More specifically, the above formulation was prepared by forming anaqueous slurry of the ceramic fibers and other inorganic fibers andorganic materials by mixing them in water (from about 0.1% to 1%consistency, and preferably 0.5% consistency). This mixing procedure iswell known in the art and is a standard technique in the industry.

After mixing, the mixture or slurry was passed through a rotoformerpapermaking machine to deliquify the composition and to form a mat offibers which essentially make up the ceramic paper. No dandy rolls orpresses were used in making the ceramic paper mat, however these may beutilized in its manufacture. The operating procedure of the papermakingmachine is also well known in the art and the technical conditionsnecessary to achieve sufficient deliquification of the mixture to formthe mat on this papermaking machine or any other type of papermakingmachine may be determined by one of ordinary skill in the art withoutundue experimentation.

Once the ceramic paper had been formed, it was then taken off-line andannealed in a high-temperature oven at approximately 1400° F. forapproximately 6 minutes. It is believed that an oven fired at least atapproximately 1000° F. will be sufficient to burn out all the organicmaterials in the ceramic paper composition. Accordingly, the essentialcomponents left as part of the ceramic paper include ceramic fibers andan inorganic binder system of fibers consisting essentially of silica.Preferably, the ceramic paper is composed of from about 50 to about 99percent by weight ceramic fibers and from about 1 to about 50 percent byweight silica.

Having produced an inorganic ceramic paper, it was subjected to thetests listed in the Table below. As indicated in the Table, the tests ofthe above ceramic papers indicated that the paper was fairly strong(tensile) for having no organic content (0% loss on ignition) and had arelatively high permeability. Ceramic papers according to the presentinvention possess strength, flexibility, porosity and permeabilityparticularly suitable for use in airbag inflator filter units, providinga flow of at least about 65 cubic feet per minute.

                  TABLE                                                           ______________________________________                                        Physical Data                                                                 Property           Data Range                                                 ______________________________________                                        Thickness (inches)                                                            @ 4 psf load       0.055 to 0.090                                             @ 8 psi load       0.020 to 0.055                                             Basis Weight (lbs/3000 sf)                                                                       100 to 200                                                 Tensile Strength (g/in)                                                                           500 to 1000                                               Fiber Index (%).sup.j                                                                            50 to 80                                                   Pressure Drop (mm H.sub.2 O)                                                                     1 to 5                                                     Flow (cfm/sf)       69 to 290                                                 Loss on Ignition (%).sup.k                                                                       0                                                          ______________________________________                                         .sup.j Amount Fiber/(Amount Fiber and Shot) (100)                             .sup.k Exclusive of water vapor                                          

Thus it should be evident that the ceramic papers of the presentinvention and its method of manufacture thereof are highly effective inproviding the necessary porosity, strength and flexibility propertiesrequired for a wide variety of applications. The invention isparticularly suited for use as a filter in the inflator unit of anautomotive airbag.

One representative form of an airbag module is indicated generally bythe numeral 10 in the drawing. It will be appreciated that all of theelements of airbag module 10 can take any form commonly known in theart, except where expressly stated herein. Thus, airbag module 10 is onecommonly used in the art. It includes a bag 12 and an inflator body 14.As shown and for ease of description, inflator body 14 is cylindrical.It should be understood, however, that the shape of inflator body 14,while conventional in the art, may or may not affect the shape anddesign of the elements included in module 10.

Positioned radially centrally within inflator body 14 is an ignitor 16which may be operatively connected to a sensor (not shown) capable ofdetecting a frontal collision. Radially outward from ignitor 16 ispressure vessel 18 containing a generant or propellant (not shown)which, when ignited, releases an inflation gas such as nitrogen. Thepropellant or generant (not shown) most typically used today is sodiumazide compressed into solid pellets. Vessel 18 preferably has aplurality of apertures 22 relatively low through its outer wail 24 topermit the gas released by the propellant to escape. The gas thentravels through an inflator filter 26 near the periphery of inflatorbody 14 and into bag 12. Filter 26 includes the ceramic paper of thepresent invention which is encapsulated in wire mesh 28 to enable thefilter paper to withstand the outburst of gas.

Accordingly, in use, as soon as a collision is detected by the sensor,an electric impulse is sent to ignitor 16 which triggers athermo-chemical reaction by igniting the propellant which, in turn,causes nitrogen or some other inflation gas to be expelled and blastedoutwardly through apertures 22 at temperatures reaching approximately2200° F. The gas, as shown by the arrows in the drawing, passes throughfilter 26 and inflates bag 12 in about 1/40th of a second. The ceramicpaper of filter 26 performs at least three functions during thisoperation. First, it entraps any unreacted particles used to form, orash resulting from the formation of, the inflation gas. Second, it coolsthe gas from its extremely high temperatures. And third, it controls theflow rate of the gas. That is, the ceramic filter paper of the presentinvention has a controlled porosity for allowing the gas to escape at acontrolled rate, thus permitting the gas to cool before entering bag 12.

Moreover, it will be appreciated that the ceramic paper of the presentinvention is inorganic and as such, does not cause outgassing or any ofthe problems associated with organic materials in filter 26 such asvariable pressure and temperature levels as described hereinabove. Thus,it should be evident that the ceramic paper of the present invention ishighly efficient and provides the porosity, strength and flexibilityproperties required for use as an inflator filter in an automotiveairbag. Less generant can be used as well as less filter material,because of its high filming efficiency. Moreover, the ceramic paper andmethod of the present invention can be used separately with otherequipment and the like in household as well as other industrialapplications, including being used in heating elements and kilns.

Based upon the foregoing disclosure, it should be apparent that the useof the ceramic paper described herein will carry out the objects setforth hereinabove. It is, therefore, to be understood that anyvariations evident fail within the scope of the claimed invention andthus, the selection of specific component elements can be determinedwithout departing from the spirit of the invention herein disclosed anddescribed. In particular, the types or grades of ceramic fiber utilizedaccording to the present invention are not necessarily limited to thosedetailed hereinabove. Moreover, as noted hereinabove, other types ofpapermaking equipment such as vacuum casters, fourdrinier machines, twinwire formers, etc., other burn-out ovens and high temperature ovens forheating the product off-line such as kilns, Rockwell ovens, etc., can besubstituted for the rotoformer papermaking machine and high-temperatureoven discussed above, respectively. Thus, the scope of the inventionshall include all modifications, variations and equivalent embodimentsthat may fall within the scope of the attached claims.

What is claimed is:
 1. A filter for use in an inflator unit of anautomotive airbag comprising an inorganic ceramic paper includinginorganic ceramic fibers and an inorganic binder of fibers consistingessentially of 100% silica, said inorganic ceramic paper being porousand free of all organic materials and having a tensile strength of atleast about 500 grams per inch wherein said fibers consistingessentially of 100% silica have an average length of about 1 to about 3inches.
 2. The filter of claim 1, wherein said ceramic paper comprisesfrom about 1 to about 50 percent by weight silica fibers.
 3. The filterof claim 1, wherein said inorganic binder initially includes organicbinder materials, but which are burned out after said ceramic paper hasbeen formed to leave said inorganic binder free of all organic bindermaterials.
 4. The filter of claim 1, wherein said ceramic paper has aflow rate of at least about 65 cubic fee per minute.
 5. The filter ofclaim 1, wherein said ceramic paper has a tensile strength of at leastabout 800 grams per inch.
 6. The filter of claim 1, wherein said ceramicpaper ranges from about 0.03 inches to about 0.25 inches in thickness.7. The filter of claim 1, wherein said ceramic fibers are selected fromthe group consisting of aluminosilicate, mineral wool, fiberglass,zirconia, zirconia tow, conventional silica, alumina and mixturesthereof.
 8. The filter of claim 1, wherein said filter is positionedradially outwardly of an ignitor located within a pressure vesselcontaining a propellant such that when said ignitor ignites saidpropellant, a reaction occurs and an inflation gas expels from saidpressure vessel and through said filter.